Process for manufacturing foil

ABSTRACT

A process is described for manufacturing dense, ductile metallic foil by vacuum evaporation. The foil is produced by vacuum evaporating and depositing the metal of the foil on a high temperature substrate. A fluoride salt release agent is used to facilitate the removal of the condensed magnetic metallic material from the substrate.

limited States Patent 1191 Kennedy et al.

1451 Apr. 24, 1973 15 1 PROCESS FOR MANUFACTURING 3,270,381 9/1966 Smith, Jr. ..164/46 FQIL 3,244,852 4/1966 Heterick et al ..l64/46 1,128,058 2/1915 Schoop ..l64/46 1 Inventors: Kurt Kennedy, Berkeley; Mau- 1,179,762 4/1916 Schoop ..164/46 rice E. Turner, El Cerrito, both of 3,631,745 I/ 1972 Walke'y et a1 ..164/46 Calif.

Primary ExaminerJ. Spencer Overholser [73] Ass1gnee. A1rco,lnc., New York, NY. Assistant Examiner v. K Rising [22] Filed: Sept. 10, 1971 Att0rneyWil1iam E. Anderson et al.

[21] Appl.No.: 179,579 [57] ABSTRACT I A process is described for manufacturing dense, duc- [52] US. Cl. ..l64/46, 164/7, 164/61, tile metallic f0 by vacuum evaporation. The foil is 164/72 117/53 106/3827 106/38'9 produced by vacuum evaporating and depositing the [51] 1111. C1. ..BZZd 23/00 metal of the f0 on a high temperature Substrate A [58] Field of Search ..l64/46, 33,61, 7, fluoride salt release agent is used to facilitate the 164/72, 65; 117/53, 106/3827, 38-9 removal of the condensed magnetic metallic material from the substrate. [56] References Cited 4 Claims, N0 Drawings UNITED STATES PATENTS 3,035,318 5/1962 Campbell ..l64/72 PROCESS FOR MANUFACTURING FOIL This invention relates generally to a process for the manufacture of dense, ductile metallic foil and, more particularly, the invention relates to the manufacture of dense, ductile metallic foil utilizing a vacuum evaporation process.

It is generally known that metallic foils may be manufactured by depositing a thin metallic coating on the surface of a substrate, and thereafter stripping the coating from the surface of the substrate in the form of a thin continuous foil sheet. As used herein, the term metal includes pure metals and metal alloys, including alloys of metals and non-metals. Foil, as distinguished from strip or plate, may be defined as a sheet of materia1 having a thickness of not more than about 6 mils.

Metallic foil may be manufactured by spraying droplets of molten metal on the surface of a substrate and allowing the metal to solidify in the form of a thin coating which is then stripped from the surface of the substrate in the form of a foil. This process is generally referred to as flame spraying. Metallic foil may also be manufactured by decomposing a gaseous metallic compound, for example, a metallic carbonyl compound, and depositing the liberated metal on the surface of a substrate in the form of a coating which is then removed from the substrate in the form ofa foil. Flame spraying and decomposition processes may be utilized to form foils of a limited number of metals, and foils produced by flame spraying and decomposition processes are generally ductile. A ductile foil, for purposes of this invention, may be defined as a foil which may be folded over and creased twice along the same line without fracturing or breaking.

Flame spraying and decomposition processes, how ever, are generally limited in the types of metals with which they are successful. For example, the decom position process cannot be employed to form foils of metals which do not form a readily decomposable gaseous compound, and cannot be employed to form foils of alloys. Flame spraying processes are not desirable for the manufacture of foils of high melting point metals and alloys since at the temperature necessary to flame spray the metal, the substrate may be impaired or destroyed. Further, flame spraying and decomposition processes do not generally provide metallic foils at sufficiently high rates to be economical in large scale operations. In addition, it is difficult to provide foils which have a high density and which are substantially free from pin holes and other imperfections in the foil when using flame spraying and decomposition processes.

Metallic foils may also be manufactured by vacuum evaporation processes, for example, as disclosed in U.S. Pat. Nos. 3,l8l,209 and 3,183,563. Metallic foil manufactured by vacuum evaporation processes are substantially free from pin holes and other imperfections, and the foil may be manufactured at a desirable rate. Vacuum evaporation processes are particularly attractive in that foil may be formed from substantially all metals and alloys, in substantially any desired thickness. The freedom from pin holes and imperfections makes foils which are manufactured employing vacuum evaporation techniques particularly desirable for such uses as electrical capacitors, where the physical properties of the foil are important. Foils suitable for such uses, however, must have good ductility, that is, the foil must be able to be manipulated and bent during manufacture.

Vacuum evaporation processes for the manufacture of foil may employ heated substrates in order to obtain the desired degree of ductility in the foil being produced. As taught by U.S. Pat. No. 3,270,381, in a vacuum evaporation process, when the substrate is maintained at a minimum temperature that is at least 25 percent of the melting point of the foil material in degrees Kelvin, the foil obtained when the deposited coating material is removed from the substrate will have good ductility, that is, the foil will be able to withstand folding and creasing along the same line at least twice without breaking or fracturing. The temperature at which the substrate is maintained in order to provide a foil product of good ductility may be as low as 25 percent of the melting point of the coating material in degrees Kelvin, or may be considerably higher. For some materials, such as titanium base alloys, it may be desirable to use substrate temperatures which exceed 50 percent of the melting point of the coating material. It is not generally necessary to exceed about 90 percent of the melting point of the coating material, particularly for those metals which have relatively high melting points, but for some metals it may be preferred to maintain the substrate at a substrate temperature above about percent of the melting point of the coating material.

Certain metal foils, such as copper, when deposited at the lower end of the aforementioned temperature range, i.e., 25 percent of the alloy melting point in degrees Kelvin, will usually be easy to separate from the substrate. Ease of separation generally means that the force necessary to withdraw the foil from the substrate will not place tension on the foil exceeding the yield point of the material. If the latter situation results, the foil may wrinkle or even rupture, resulting in an unsatisfactory product. Under some circumstances, however, the foil may not readily release from the substrate, and the aforementioned wrinkling or rupture of the foil may result. In order to facilitate foil release, various types of release agents may be used.

At substrate temperatures exceeding about 800 Kelvin, satisfactory release agents may be difficult to find. Organic compounds such as teflon, polyethylene, silicones, etc., are often unstable and decompose at such temperatures. Oxides and various metal salts may be unstable, particularly in the presence of alloys containing reactive elements such as titanium, niobium, and the like.

It is an object of the present invention to provide an improved process for the manufacture of dense, ductile metallic foil by vacuum evaporation.

Another object of the invention is to provide a vacuum evaporation foil making process in which high substrate temperatures are used and in which the foil readily releases from the substrate.

A further object of the invention is to provide a process for the manufacture of foil by vacuum evaporation which employs a release agent capable of satisfactorily functioning despite high substrate temperatures and reactive elements in the foil.

Other objects of the invention will become apparent to those skilled in the art from the following description.

Very generally, the invention provides a process for the manufacture of dense ductile metallic foil by vacuum evaporation. A fluoride salt of an element selected from the group consisting of lithium, magnesium, calcium, strontium and barium is applied to a substrate in a layer between 500 and 2000 angstrom units thick. The substrate is heated to a temperature of at least about 800 Kelvin. A metal-bearing coating material is vaporized within a vacuum and at least a portion of the metallic content of the vaporized coating material is condensed on the salt layer on the substrate. During condensation, the temperature of the substrate is maintained at least above 800 Kelvin. The substrate and its coating are subsequently cooled and the condensed metallic material is removed from the substrate.

The method of the invention may be carried out in a conventional vacuum evaporation chamber. The coating material from which it is desired to form a dense ductile foil is placed within a crucible or container within the vacuum chamber and is heated to a temperature at which the coating material vaporizes at an ap preciable rate. The vacuum chamber is desirably maintained at a pressure below about 1 millitorr, preferably below about 0.1 millitor, in order to provide for a suitable flow rate of vaporized coating material. The vacuum chamber may be evacuated utilizing conventional vacuum diffusion pumps, and the coating material may be vaporized by a suitable heat source. One form of heat source that is particularly suited for use in vacuum evaporation processes is an electron beam gun, such as is more fully described in U.S. Pat. No. 3,177,535.

A substrate is positioned above the crucible containing the coating material and the vaporized coating material is deposited upon the surface of the substrate in the form of a thin uniform coating. For batch operations the substrate may be stationary, and for continuous operations the substrate may be in the form of an endless belt, or the like, which is moved slowly over the open mouth of the crucible. It is also possible to form the substrate in the form of a cylindrical drum which may be mounted in the wall of the vacuum chamber as disclosed in U.S. Pat. No. 3,183,563. Any suitable material may be employed as a substrate. A desired substrate is one that has a non-porous smooth surface, for example, stainless steel. The coating material is then removed from the surface of the substrate in the form of a foil.

The substrate may be heated to above the minimum temperature desired in order to obtain ductile foil by a suitable heat source positioned adjacent thereto. It is possible to employ induction heating adjacent the back surface of the substrate, or the surface of the substrate may be exposed to a radiant heater or an electron beam gun prior to the coating step. In some instances, it may also be desirable to provide means for cooling the substrate should the temperature of the substrate exceed the desired amount.

The foil may be removed from the surface of the substrate within the vacuum chamber, or the substrate containing the coating material deposited thereon may be passed out of the vacuum chamber and the foil removed from the substrate exteriorly of the vacuum chamber. The foil may be removed from the substrate by pulling the foil under tension from the surface of the substrate. This may be accomplished by means of a take-up roll or drum angularly spaced from the substrate about which the foil is wound, and which is rotated at a slightly greater linear velocity than the velocity of the substrate, causing the foil to be lifted tangentially from the surface of the substrate. lt is also possible to utilize a doctor blade or the like in order to aid in the separation of the foil from the surface of the substrate.

In order to provide satisfactory release of the foil from the substrate, such that tension applied to the foil during removal does not exceed its yield point, the process of the invention utilizes a release agent. Preferably, the release agent is applied to the substrate prior to condensing the metal thereon by evaporating the release agent from a molten source contained in a suitable crucible or boat. Heating of the release agent to its evaporation temperature may be accomplished by any suitable means, such as resistance heating, or electron beam heating. Other means of applying the release agent to the substrate may also be satisfactory.

The thickness to which the release agent is deposited on the substrate should be at least about 500 angstrom units thick. This is to insure that a sufficient amount of the release agent will be present to facilitate release. Thicknesses up to 2000 angstrom units are possible, but beyond that thickness problems result from prerelease. Pre-release typically involves a sagging or otherwise premature separation of the foil from the substrate. The best results have been achieved by utilizing a thickness of about 800 angstrom units to about 1600 angstrom units for the release agent. This preferred thickness range may typically be achieved by utilizing deposition rates of between 0.025 and 0.050 grams per square foot of substrate.

The release agent comprises a fluoride salt. More particularly, the fluoride salt used should have a boiling point well in excess of the preheat temperature of the substrate. The group of fluoride salts which perform satisfactorily consists of fluoride salts of lithium, sodium, manganese, calcium, strontium and barium. Lithium fluoride and sodium fluoride may be unsatisfactory above about ll00 Kelvin, but the remaining fluoride salts in the above group are satisfactory over the entire range up to 1400 Kelvin. Since calcium fluoride is the most readily obtainable fluoride salt, occurring in nature as the mineral fluorospar, calcium fluoride is preferred as the release agent. As previously mentioned, the maximum temperature at which a release agent is satisfactory is related to its boiling point. The fluoride salts of manganese, calcium, strontium, and barium have boiling points of between 2400 and 2500 Kelvin and therefore may be expected to have better stability at temperatures exceeding ll00 Kelvin than lithium fluoride and sodium fluoride which have lower boiling points.

The fluoride salt release agent may be melted and evaporated onto the substrate from warm pressed pure powder bars, hot pressed pure powder bars, or melted and cast pure powder bars. The melted and cast bars are preferable from the standpoint of thermal shock and uniform evaporation rate. The purityof the release agent should be very high, preferably above 99 percent. Satisfactory results have been obtained at a purity of 99.9 percent.

remove-the release agent from both the foil and the substrate prior to commercial application of the foil and re-use of substrate.

vaporizing at a reduced pressure a fluoride salt of an element selected from the group consisting of lithium, calcium, sodium, magnesium, strontium, and barium, condensing the fluoride salt on a substrate in a layer between 500 and 2000 angstrom units thick, vaporizing a metal-bearing coating material within a vacuum, condensing at least a portion of the metallic content of the vaporized coating material on the salt layer on the substrate, and removing the condensed metallic material from the substrate.

2. A process according to claim 1 wherein the substrate is heated, during condensation of the metallic content, to and maintained at a temperature at least EXAMPLES Melting Substrate point, Substrate tempera- Thickness. Example Foil material K. material ture, K. Release agent angstroms 'Ii-l1A1-4V 1,900 Stainless steel... 800-1, 000 Lithium fluoride 800-1, 600 Ti-tiAl-iv 1, J00 do 1, 000-1, 170 Calcium fluoride 800-1, 600 'Ii-fiAl-4V 1, 200-1, 300 Strontium fiuorid 500-2, 000 Ti-6Ai-4V 1, 200-1, 300 Barium fluoride. 500-2,000 Ti-6A1-4V 1, 000-1, 200 Magnesium iluori 500-2, 000 'ii-5Al-2.5Sn 000-1, 000 Calcium fluoride 800-1, 600 '1i-3A 1-2.5V "00-1, 100 800-1, 000 Ni-lllGr-itlSi 800-1, 000 Ni-Cr-5Al 1, 300-1, 350 800-1, 000 (ii-BMH-llNi U00 "(l0 8001, 000 (Ju-23itln-0Ni J00 Sodium fluoride $004,000

It may therefore be seen that the invention provides an improved process for the manufacture of dense ductile metallic foil by vacuum evaporation. High substrate temperatures necessary for the production of ductile foil in many materials are achievable while at the same time satisfactory release of the foil from the substrate after coating is accomplished.

Various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. A process for the manufacture of dense ductile metallic foil by vacuum evaporation comprising,

about 800 Kelvin, and less than about 1400 Kelvin, and wherein the fluoride salt is of a material selected from the group consisting of magnesium, calcium, strontium and barium.

3. A process according to claim 1 wherein the substrate is heated, during condensation of the metallic content, to and maintained at a temperature within the range from about 800 Kelvin to l400 Kelvin, and wherein the fluoride salt comprises calcium fluoride.

4. A- process according to claim 1 wherein the substrate is heated during condensation of the metallic content, to and maintained at least about 1000 Kelvin, and wherein the fluoride salt is of a metal selected from the group consisting of magnesium, calcium strontium and barium. 

2. A process according to claim 1 wherein the substrate is heated, during condensation of the metallic content, to and maintained at a temperature of at least about 800* Kelvin, and less than about 1400* Kelvin, and wherein the fluoride salt is of a material selected from the group consisting of magnesium, calcium, strontium and barium.
 3. A process according to claim 1 wherein the substrate is heated, during condensation of the metallic content, to and maintained at a temperature within the range from about 800* Kelvin to 1400* Kelvin, and wherein the fluoride salt comprises calcium fluoride.
 4. A process according to claim 1 wherein the substrate is heated during condensation of the metallic content, to and maintained at least about 1000* Kelvin, and wherein the fluoride salt is of a metal selected from the group consisting of magnesium, calcium, strontium, and barium. 